1. Field of the Invention
This invention relates to panels for electrical switching apparatus and, more particularly, to panels for circuit breakers, such as, for example, aircraft circuit breakers. The invention also relates to methods of electrically grounding electrical switching apparatus, such as circuit breakers.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition between a power source (e.g., a line terminal) and a load.
Subminiature circuit breakers are used, for example, in aircraft electrical systems where they not only provide overcurrent protection but also serve as switches for turning equipment on and off. As such, they are subjected to heavy use and, therefore, must be capable of performing reliably over many operating cycles. They also must be small to accommodate the high-density layout of circuit breaker panels, which make circuit breakers for numerous circuits accessible to a user. Aircraft electrical systems usually consist of hundreds of circuit breakers, each of which is used for a circuit protection function as well as a circuit disconnection function through a push-pull handle.
Typically, subminiature circuit breakers have only provided protection against persistent overcurrents implemented by a latch triggered by a bimetal responsive to I2R heating resulting from the overcurrent. There is a growing interest in providing additional protection, and most importantly arc fault protection. Arc faults are typically high impedance faults and can be intermittent. Nevertheless, such arc faults can result in a fire.
Many non-aircraft circuit breakers employ ground fault protection. In aircraft applications, the aircraft frame is ground, and there is no neutral conductor. Some aircraft systems have also provided ground fault protection, but through the use of additional devices, namely current transformers which in some cases are remotely located from the protective relay.
Typically, aircraft circuit breaker panels are, at best, poor conductors (e.g., such panels are painted; are made of a non-conductive composite material; or are made of an oxidized conductive material, such as aluminum).
In order to monitor faults, such as arc faults in aircraft circuit breakers, there exists the need to power arc fault detection circuitry. Hence, there exists the need to provide a reliable ground connection to the aircraft circuit breaker in addition to the existing line terminal from the power source.
U.S. Pat. No. 5,527,991 discloses a U-shaped metal grounding strap for a panel-mounted electrical switch. The grounding strap includes two legs having serrated segments, which scrape the edges of an opening in the panel in order to remove any paint or non-conductive coating on the panel.
U.S. Pat. No. 4,039,235 discloses a grounding strip for an electrical receptacle. The grounding strip includes an extension having screw-engaging means underlying an opening in the mounting ears of the receptacle. Metal screws, in turn, provide a self-grounded connection through the screw-engaging means to a grounded wall box.
There is room for improvement in circuit breaker panels and methods of electrically grounding circuit breakers.
The present invention employs an electrically conductive grounding member, such as a grounding strip which is adapted for electrical connection to ground, proximate the surface of a circuit breaker panel. A circuit breaker has an electrically conductive surface, which electrically engages the electrically conductive grounding member.
According to one aspect of the invention, a method of electrically grounding a circuit breaker comprises: employing a panel having a surface; grounding an electrically conductive grounding member; employing a circuit breaker having an electrically conductive surface; employing the electrically conductive grounding member between the surface of the panel and the electrically conductive surface of the circuit breaker; mounting the circuit breaker to the panel; and electrically engaging the electrically conductive grounding member with the electrically conductive surface of the circuit breaker.
A circuit breaker may be employed having a bezel and a pair of mounting holes; a first opening may be employed in the panel corresponding to the bezel of the circuit breaker and a pair of second openings may be employed in the panel corresponding to the mounting holes of the circuit breaker. The bezel of the circuit breaker may be passed through the first opening in the panel. The circuit breaker may be fastened to the panel with a pair of fasteners, which engage the panel at the second openings thereof and the circuit breaker at the mounting holes thereof.
The pair of second openings may be employed in the panel on opposite sides of the first opening thereof. A first opening may be employed in the electrically conductive grounding member corresponding to the bezel of the circuit breaker and a pair of second openings may be employed in the electrically conductive grounding member on opposite sides of the first opening thereof. The bezel may be passed through the first opening of the electrically conductive grounding member.
The electrically conductive grounding member may be a silver-plated copper strip.
Preferably, the electrically conductive surface of the circuit breaker is a mounting plate having a raised surface, and the electrically conductive grounding member is electrically engaged with the raised surface of the mounting plate. Preferably, the electrically conductive grounding member is sandwiched between the panel and the mounting plate of the circuit breaker, thereby electrically engaging the electrically conductive raised surface of the circuit breaker with the electrically conductive grounding member.
The circuit breaker may have a bezel with an electrically conductive engagement surface as the electrically conductive surface of the circuit breaker, and the electrically conductive grounding member may be electrically engaged with the electrically conductive engagement surface. Preferably, the electrically conductive grounding member is sandwiched between the panel and the electrically conductive engagement surface, thereby electrically engaging the electrically conductive surface of the circuit breaker with the electrically conductive grounding member.
As another aspect of the invention, a circuit breaker panel comprises: a panel having a surface; an electrically conductive grounding member adapted for electrical connection to ground; a circuit breaker having an electrically conductive surface; and at least one fastener fastening the circuit breaker to the panel, with the electrically conductive grounding member between the surface of the panel and the electrically conductive surface of the circuit breaker, and with the electrically conductive surface of the circuit breaker electrically engaging the electrically conductive grounding member.